A wide range of vehicles and methods are known for transporting human subjects. Typically, such vehicles rely upon static stability and are designed for stability under all foreseen conditions of placement of their ground-contacting members with an underlying surface. For example, a gravity vector acting on the center of gravity of an automobile passes between the points of ground contact of the automobile's wheels and the suspension of the automobile keeps all wheels on the ground at all times making the automobile stable. Although, there are conditions (e.g., increase or decrease in speed, sharp turns and steep slopes) which cause otherwise stable vehicles to become unstable.
A dynamically stabilized vehicle, also known as a balancing vehicle, is a type of vehicle that has a control system that actively maintains the stability of the vehicle while the vehicle is operating. In a vehicle that has only two laterally-disposed wheels, for example, the control system maintains the fore-aft stability of the vehicle by continuously sensing the orientation of the vehicle, determining the corrective action necessary to maintain stability, and commanding the wheel motors to make the corrective action. If the vehicle losses the ability to maintain stability, such as through the failure of a component or a lack of sufficient power, the human subject can experience a sudden loss of balance.
For vehicles that maintain a stable footprint, coupling between steering control and control of the forward motion of the vehicles is less of a concern. Under typical road conditions, stability is maintained by virtue of the wheels being in contact with the ground throughout the course of a turn and while accelerating and decelerating. In a balancing vehicle with two laterally disposed wheels, however, any torque applied to one or more wheels affects the stability of the vehicle.
In prior art systems, such as the self balancing vehicles shown in U.S. Pat. No. 5,871,091 personal vehicles may be self-propelled and user-guidable, and, further, may entail stabilization in one or both of the fore-aft or left-right planes, such as when no more than two wheels are in ground contact at a time. Vehicles of this sort may be operated in a mode in which motion of the vehicle, including acceleration (both linear and turning), is commanded partially or entirely by leaning of the vehicle as caused by a subject riding the vehicle. Several such vehicles are described in U.S. application Ser. No. 08/384,705 which is incorporated herein by reference.
Such balancing vehicles may lack static stability. Referring, for example, to FIG. 1A, wherein a prior art personal transporter is shown and designated generally by numeral 18, a subject 10 stands on a support platform 12 and holds a grip 14 on a handle 16 attached to the platform 12, so that the vehicle 18 of this embodiment may be operated in a manner analogous to a scooter. A control loop may be provided so that leaning of the subject results in leaning of the platform which, in turn, results in the application of torque to wheel 20 about axle 22 thereby causing an acceleration of the vehicle. Vehicle 18, however, is statically unstable, and, absent operation of the control loop to maintain dynamic stability, subject 10 will no longer be supported in a standing position and can fall from platform 12. Another prior art balancing vehicle is shown in FIG. 1B and designated generally by numeral 24. Personal vehicle 24 shares the characteristics of vehicle 18 of FIG. 1A, namely a support platform 12 for supporting subject 10 and grip 14 on handle 16 attached to platform 12, so that the vehicle 24 of this embodiment may also be operated in a manner analogous to a scooter. FIG. 1B shows that while vehicle 24 may have clusters 26 each having a plurality of wheels 28, vehicle 24 remains statically unstable and, absent operation of a control loop to maintain dynamic stability, subject 10 will no longer be supported in a standing position and may fall from platform 12.
A standing rider 10 of the vehicle 30 places his feet on the platform and shifts weight back and forth in a relatively wide and flat path 33. The slight amount of strength that is needed to resist gravity and inertia in transversing this arc is well within the strength and coordination of an average user's muscles. The center of gravity of the vehicle and rider 35 moves in an arcuate fashion as the rider leans either forward or backward. When a seat is added to such a vehicle, movement of the center of gravity in the manner described above may no longer be possible and an alternative mechanism for shifting the center of gravity is required. The mechanism needs to provide adequate range of motion while allowing the rider to resist gravity and inertia.